1. Field of the Invention
The present invention relates to a line illuminating device for linearly illuminating a document in a document-reading device such as a contact-type image sensor.
2. Description of the Prior Art
Equipment such as a facsimile machine, a copying machine and an image scanner is provided with a document-reading device (an image reading device) such as an image sensor as a device for reading a document. One type of the document-reading device used is a contact-type image sensor of which the optical path length is short and which can be easily incorporated in the equipment. In this contact-type image sensor, it is necessary to read the portion to be read of the document by applying light in excess of the intensity of illumination readable by an illuminating device. In this case, the range to be illuminated is quite large in a main-scanning direction (i.e. in the longitudinal direction), but is a narrow, belt-like strip in a sub-scanning direction at right angles to the main-scanning direction.
A line illuminating device using a bar-shaped or plate-shaped light guide for illuminating the above-mentioned long and narrow belt-like range and a document-reading device provided with such a line illuminating device are disclosed in Japanese Unexamined Patent Publication No. Hei 8-163320 or Japanese Unexamined Patent Publication No. Hei 10-126581.
FIG. 24 is a cross-sectional view of a document-reading device which is provided with a conventional line illuminating device. FIG. 25 is an exploded perspective view of the conventional line illuminating device and FIG. 26 is a perspective view showing the structure of a light guide forming the conventional line illuminating device.
The conventional document-reading device comprises, as shown in FIG. 24, a frame 501 in which recesses 501a and 501b are formed, a line illuminating device 510 disposed in the recess 501a, a base plate 504 on which a photoelectric conversion element (a line image sensor) 503 is mounted attached to cover the recess 501b, and a rod lens array 505 held in the frame 501. Light emitted from an emission plane 511a of the line illuminating device 510 is incident on a reading image surface of a manuscript 507 through a cover glass 506 of a document stand. The reflected light is then detected by the photoelectric conversion element 503 through the rod lens array 505 to read the document.
In the line illuminating device 510 as shown in FIG. 25, a light guide 511 is housed in a white casing 512 in such a manner that the emission plane 511a is exposed. Attached to one end of the casing 512 is a light-emitting source base plate 513 which is provided with a light-emitting source consisting of a light-emitting diode (LED) or the like. The light guide 511 is made of glass or transparent resin and is basically rectangular in its cross-section in the direction at right angles to a main-scanning direction (i.e. the longitudinal direction). The light guide 511 has a chamfered C-shaped portion which serves as the emission plane 511a. 
As shown in FIG. 26, the light guide 511 is provided, on its reverse surface 511c, with light-scattering patterns 511b for scattering the illuminating light incident from an incident plane 511d. The light-scattering patterns 511b are formed by screen printing a white coating material.
This line illuminating device 510 is arranged in such a manner that light from the light-emitting source such as an LED is introduced from one end (the incident plane) 511d of the light guide 511 into the inside of the light guide 511, the illuminating light spreading through the light guide 511 is scattered at the light-scattering patterns 511b which are formed on the reverse surface 511c of the light guide 511, and then this scattered light is emitted from the emission plane 511a (see FIG. 24).
Intensity of light incident from the light-emitting source is high near the incident plane 511d, while the intensity of light becomes lower as the light-scattering patterns recede from the incident plane. Now, as shown in FIG. 26, it is intended that the intensity of light emitted from the emission plane 511a be uniform over the full length of the main-scanning direction by broadening a forming area of the light-scattering patterns 511b as the light-scattering patterns recede from the incident plane 511d. 
As described above, in the conventional line illuminating device 510 as shown in FIGS. 24 through 26, it is intended to provide uniform distribution of the light intensity in the main-scanning direction by shaping of the forming area of the light-scattering patterns 511b which are provided on the light guide, but it is difficult to make the distribution of light intensity completely uniform in the main-scanning direction. Thus, the distribution of light intensity in the main-scanning direction in the conventional line illuminating device often shows a tendency to monotonic increase or monotonic decrease. It is also difficult to make the distribution of light intensity uniform in a sub-scanning direction.
Further, the conventional line illuminating device is constructed, as shown in FIG. 24, to apply illuminating light to the document surface from one direction, and, as a result, if there is any fold or laminating difference in level on the document paper surface, shading may be caused.
FIG. 27 is a cross-sectional view of an other conventional contact-type image sensor and FIG. 28 is a perspective view (showing the condition in which the light guide casing is removed) of the conventional line illuminating device which is used in the contact-type image sensor as shown in FIG. 27. FIG. 29 is a view showing the intensity distribution of light emitted from the line illuminating device as shown in FIG. 28. FIG. 30 is a perspective view showing the positional relationship between the line illuminating device and an image-formation lens in the contact-type image sensor as shown in FIG. 27. FIG. 31 is a plan view showing the positional relationship between the line illuminating device and the image-formation lens in the contact-type image sensor as shown in FIG. 27.
As shown in FIG. 27, the conventional contact-type image sensor 601 comprises a box 602, a line illuminating device 610 incorporated in the box 602, a lens array 605 arranged in the box 602 to serve as the image-formation lens, and a base plate 607 on which a line image sensor 606 (a photoelectric conversion element) is mounted attached to the lower section of the box 602. The line illuminating device 610 comprises a light guide 603, a light guide casing 604 and a light-emitting source base plate 611 as shown in FIG. 28. According to this contact-type image sensor 601, light (illuminating light) emitted from an emission plane 603a of the light guide 603 is incident on a reading surface of a document through a cover glass 608. The reflected light is detected by the line image sensor 606 through the lens array 605 to read the document.
As shown in FIG. 28, the line illuminating device 610 consists of the light guide 603, the light-emitting source base plate 611 provided with a light-emitting source 612 such as a light-emitting diode (LED), and the light guide casing 604 (not shown). FIG. 28 shows the condition in which the light guide casing 604 is removed. The light guide 603 is, as shown in FIG. 27, housed in the white light guide casing 604 in such a manner that the emission plane 603a is exposed. The light guide 603 is made of glass or transparent resin. Shown here is one example of the light guide 603 which has a cross-sectional shape substantially ¼ oval in the direction at right angles to the longitudinal direction. This light guide 603 is provided with an emission plane 603a parallel to the direction of a minor axis of the oval, a plane 603b parallel to the direction of a major axis of the oval, and a reflecting curved surface 603c. Scattering patterns P are formed on the plane 603b parallel to the major axis direction of the oval by printing white coating materials and the like.
The light emitted from the light-emitting source 612 spreads through the inside of the light guide 603 and a part of the light is scattered at the light-scattering patterns P on the light guide surface formed by the white coating materials and the like. The scattered light is reflected at the reflecting curved surface 603c to be radiated to the outside from the emission plane 603a as emitted light.
By covering the light guide 603 with the light guide casing 604, the light emitted outside is caused to reflect at the light guide casing 604 to return it to the inside of the light guide 603. Thus, loss of scattered light is reduced and as a result, intensity of the emitted light is improved. The light-scattering patterns P are formed at a position near the focal point of the plane 603b parallel to the major axis direction of the oval. With this, the light scattered by the light-scattering patterns P is reflected at the reflecting curved surface 603c and is concentrated on the document-reading surface. As a result, it is possible to improve the intensity of light on the document-reading surface.
As shown in FIG. 29, each light intensity distribution Ba˜Bf is formed by each light-scattering pattern Pa˜Pf. The light intensity distribution C of FIG. 29 shows a result that each intensity distribution Ba˜Bf from each light-scattering pattern Pa˜Pf is propagated by a distance L0 and then has been compounded, while the light intensity distribution D shows a result in which each intensity distribution Ba˜Bf from each light-scattering pattern Pa˜Pf is further propagated by a distance ΔL0 and has been compounded.
As shown in FIG. 30, an X-Y plane (the surface of the cover glass 608 as shown in FIG. 27) is situated immediately above an optical axis of the lens array 605. The paper surface (the reading surface of the document) is illuminated by the emitted light from the light guide 603 of the line illuminating device 610. The light intensity distribution of the irradiated light in the X-Y plane exhibits the characteristics shown by the reference mark C in FIG. 29.
As shown in FIG. 29, in the line illuminating device 610 in which the light-scattering patterns Pa˜Pf are intermittently formed over the longitudinal direction (the main-scanning direction) of the light guide 603, unevenness corresponding to the intervals between the light-scattering patterns Pa˜Pf.is caused in the light intensity distribution of the emission light. The unevenness of the light intensity distribution results in unevenness of the reading image. It is therefore desired to provide the line illuminating device with less unevenness in the light intensity distribution.
On the other hand, sensitivity of the line image sensor 606 also varies for each picture element. Accordingly, it may be necessary to correct, by image processing (what is called shading correction), the unevenness of the light intensity distribution of the line illuminating device and variation of sensitivity in the line image sensor. Specifically, an image is read in such a condition that for example, a white document is closely applied to the surface of the cover glass 608 and an output for each picture element of the image sensor 606 is determined. Based on the output for each picture element, a total correction amount (or correction factor) of the unevenness of the intensity distribution of the light source and the variation of sensitivity for each picture element of the image sensor 606 is computed. The computed correction amount is stored for each picture element. When the document is read, the above-mentioned correction amount is corrected for the output of each picture element of the image sensor 606. With this, it is possible to solve the unevenness of the intensity distribution of the light source and the variation of sensitivity for each picture element of the image sensor 606.
However, due to the fold or backward bending of the document, there is some possibility that the reading surface of the document is elevated from the X-Y plane (the cover glass surface). When the document surface is displaced due to the elevation of the document and the like, amplitude of the unevenness of the light intensity distribution (the amplitude of the swell of characteristics as shown by the reference mark C) changes in addition to an entire change of quantity of light. As a result, unevenness is caused in the reading image.
Accordingly, it is desired to provide a line illuminating device which can provide a uniform light intensity distribution in the main-scanning direction and in which unevenness is not easily produced on the reading image even if the distance between the document surface and the light source changes because of elevation of the document surface.